1. Field of the Invention
The invention concerns a hydraulic drive, in particular for a two-cylinder thick matter pump of the general type described in the pre-characterizing portion of claim 1.
2. Description of Related Art
Known hydraulic drives of this type include at least one main pump embodied as a hydraulic variable displacement pump as well as two hydraulically actuated drive cylinders embodied as piston cylinder units. The drive cylinders are connected at their one end via pump connections with respectively one of two connection lines of the main pump, forming a closed hydraulic circuit, while on their end opposite to the pump connections they are in communication with each other via an oscillating hydraulic line. The main pump, which is embodied as a reciprocating pump, is further connected with a control mechanism for alternatingly reversing the direction of flow with reciprocal building up of high pressure and a pre-tensioned low pressure in the two connection lines. Further, a hydraulic feed pump is provided, of which the suction inlet is connected to a hydraulic oil tank and the pressure outlet is set to a predetermined low pressure level and communicates with the two connection lines of the main pump via respectively one one-way valve. Since the hydraulic oil heats up during the pumping process, a sump or flushing branch is supplementally provided, which on the outlet side communicates with the oil tank via a pressure limiting valve and on the inlet side is respectively connectable with the low pressure part of the hydraulic circuit. For this purpose there is located in the flushing branch a reciprocating valve pre-controlled by the pressure differential existing between the connection lines of the main pump, which in the case that there is a prevailing pressure differential, directs flow towards the respective low pressure side connection pipe which is accompanied by discharging of a sump stream into the oil sump tank, and in the case that there is no pressure differential is in a blocked intermediate position. The amount of oil discharged during the flushing process corresponds to approximately 50-70% of the oil amount continuously re-supplied from the oil tank by the feed pump. Due to the mass inertia and the compressibility in the system, substantial pressure oscillations result within the hydraulic system during the reversal process of the reversible pump. During the reversing process the pivot angle of the adjustable pump is retracted. Thereby the volumetric displacement of the reversible pump becomes lower. Since the system is still on line, the high pressure drops while in equal value the low pressure increases. This means that the low pressure side experiences a rapid pressure increase so long as the reverse valve in the flushing cycle is not yet redirected. This leads to an extreme discharge of flushing oil from the until now low pressure side of the main circuit so that, in the course of the renewed pressure buildup on the high pressure side, the pressure on the low pressure side can completely collapse within a fraction of a second. The feed pump in this condition is not able to compensate for and re-supply the flushed amount discharged from the flush circuit. Due to the undersupply on the low pressure side, pipe clanging and cavitation is produced both in the feed pump as well as in the main pump, with a danger of an increased wear and tear. In order to avoid this problem, it has already been proposed that the deficit in oil is to be compensated by a pressure reservoir or a larger feed pump. Both solutions however require an undesirably high construction investment.